Radio Frequency Identification (RFID) technology is one kind of Automatic Identification and Data Capture technologies. RFID technology generally involves interrogating an RFID tag with radio frequency (RF) waves and reading the responding RF waves with a RFID reader. A RFID tag typically includes a miniscule microchip coupled to an RF antenna. RFID tags can be attached to the object to be identified. An RFID reader typically includes an antenna coupled to a transmitter and a receiver.
FIG. 1 shows a part of a simplified RFID reader in one specific kind of implementation. In FIG. 1, the RFID reader includes an antenna 90 coupled to a transmitter 80 and a low noise amplifier 60. The RFID reader also includes a three-port circulator 50, a demodulator 70, and a frequency generator 40. The transmitter 80 can include a power amplifier (PA), and the frequency generator 40 can include a phase-licked-loop (PLL). The three-port circulator 50 includes a port 51, a port 52, and a port 53.
In operation, the transmitter 80 generates an RF interrogation signal. This RF interrogation signal is coupled to the antenna 90 through the three-port circulator 50. The electromatic waves radiated from the antenna 90 are then received by the antenna in an RFID tag. In response to the interrogation from the RFID reader, the RF tag will reflect responding electromagnetic waves coded with the identification information of the RF tag. The responding electromatic waves are picked up by the antenna 90 as a responding RF signal. The responding RF signal enters the port 52, leaves the port 53, and is received by the low noise amplifier 60. The RF signal received by the receiver, after amplification, is demodulated with demodulator 70 that receives a reference RF signal from the frequency generator 40. The demodulated signals from the demodulator 70 is coupled to certain signal processing circuit to decode from the demodulated signal the identification information returned by the RF tag. In some implementations, the demodulator 70 is a dual quadrature demodulator, and the demodulated signals from the demodulator 70 can be a demodulated vector signal that includes two components, the in-phase demodulated signal Irx and the quadrature demodulated signal Qrx. This demodulated vector signal can be coupled to certain signal processing circuit for further signal processing.
In an ideal situation, the low noise amplifier 60 should only receive the responding RF signal generated by the RF tag that is coupled from the port 52 to the port 53. In reality, however, the low noise amplifier 60 also receives other RF signals generated from other sources or propagation paths. If the magnitude of these other RF signals are much larger than that of the responding RF signal generated by the RF tag, it will increase the difficulty for decoding the responding RF signal in order to obtain the identification information returned by the RF tag. Unfortunately, there are several other sources or propagation paths to generate these other RF signals for the simplified design of the RFID reader as shown in FIG. 1. First, even though majority of the RF interrogation signal generated by the transmitter 80 will be coupled to the port 52, some small fraction of the RF interrogation signal can still be coupled to the port 53 and received by the low noise amplifier 60. Second, because of possible impedance mismatch between the antenna 90 and the port 52, even for those RF interrogation signal transmitted to the port 52 form the port 51, some fraction of it can still be reflected back from the antenna 90, enter the port 52 and be coupled to the port 53. Third, when electromatic waves are radiated from the antenna 90, they are not just received by the antenna in the RFID tag, some fraction of these electromatic waves can be reflected from the objects nearby the RFID tag. These reflected electromatic waves can follow almost the same propagation path as that followed by the responding electromatic waves generated by the RF tag. Consequently, these reflected electromatic waves can generate some RF signals at the port 53.
To improve the signal quality of the responding RF signal generated by the RF tag as received by the receiver in the RF tag reader, it is desirable to minimize the RF signals at the input of the receiver which are generated from sources other than the RF tag.